Infections caused by such intracellular pathogens as mycobacteria, Salmonella, and Listeria annually claim millions of lives worldwide. This proposal develops the fruit fly, Drosophila melanogaster as a model organism to study these infections. Mutations in both the host and pathogens will be tested. The fruit fly has been used previously to study innate immune responses to gram positive and negative bacteria as well as fungi. Past work focused only on microorganisms that grow freely in the extracellular space. This proposal describes experiments that will use Drosophila to study intracellular infections of macrophages. Growth characteristics including growth rate, lethality and location will be measured for several bacteria that cause infections of Drosophila macrophages (Mycobacterium marinum, Salmonella typhimurium, Listeria monocytogenes). Drosophila mutants expected to have difficulty fighting intracellular pathogens will be tested for their ability to defend against intracellular infections. Salmonella typhimurium mutants will be tested for their ability to infect the fly, to determine the role bacterial genes play in virulence. These experiments will provide tools to analyze a collection of Drosophila mutants isolated based upon a phenotype that is likely to involve the cellular immune response.